Introduction to Genetics and Evolution is a college-level class being offered simultaneously to new students at Duke University. The course gives interested people a very basic overview of some principles behind these very fundamental areas of biology. We often hear about new "genome sequences," commercial kits that can tell you about your ancestry (including pre-human) from your DNA or disease predispositions, debates about the truth of evolution, why animals behave the way they do, and how people found "genetic evidence for natural selection." This course provides the basic biology you need to understand all of these issues better, tries to clarify some misconceptions, and tries to prepare students for future, more advanced coursework in Biology (and especially evolutionary genetics). No prior coursework is assumed.

Taught By

Dr. Mohamed Noor

Transcript

Hello, and welcome back to Introduction to Genetics and Evolution. In the previous video we talked about how a greater variance in mating success for males made it so sexual selection was typically more intense on males than on females. That, typically speaking, in many animal species, or at least in a stereotyped animal species, each female would be mated, but there would be this tons of variance where a lot of males never mate and a couple of males were very, very successful at mating. So what this did is it created an opportunity for two different forms of sexual selection, both of which were recognized by Darwin. That of competition among males and that for choice by females for particular kinds of males. So let's look at each of these in turn. Let's start with male competition. So how is it that males compete amongst each other for access to females? Well, I'll talk about three brief types. One is direct combat and things related to that such as forming hierarchies. The second being territoriality. And the last looking at particular strategies males can employ. It may be being sneaky in some ways to get access to females. Now the most obvious form of sexual selection is direct combat amongst males, right? Or, now essentially it's not uncommon for males to battle for access to females. In fact we know this from our own cultural references. Maybe you're familiar with the story of Helen of Troy. That's a classic example of a very long battle among males for access to a particular female. But we see this in all sorts of other species as well. Insects, birds, and other mammals etc. Now these battles, although not uncommon, tend to be very costly. It's not good for you to injure yourself in the process of trying to stake a claim for a female or a set of females. So what a lot of animals have done to compensate for this. Again, not thinking this through but just evolutionarily this has happened, is they compete without actually fighting to the death or damage. They have these sorts of displays. And then if it looks like this animal is stronger than that animal, based on this communication signal display, the other one backs off and says, all right, fine, you've won. Now related to this, a lot of social species establish what's referred to as dominance hierarchies. Now this is very different from the genetic dominance we talked about. In this case we are talking about behavioral dominance. And basically somebody is the alpha male. Somebody is the top dog among this group. So if you have a set of males, there is one who is clearly recognized as being the top, another one may be recognized as being second, etc. Now what may happen is the top male that has access to whatever female he wants, he will frequently be challenged by these other males. They'll have these fights sometimes, maybe suggesting some reordering of the hierarchy. But by and large everybody acknowledges this is the top male. There's advantages to this, because, again, you're not fighting to the death, you're not having this sort of frequent injury and things like that. By having this hierarchy exist, everybody just knows what's supposed to happen. Now, a related thing, or alternative to combat dominance is to delineate and defend a particular territory. In this case, males will try to keep other males away from their mates or their resources. And then they do this, they may try to signal others to say away through vocalizing as you can see this bird is doing here as well as this elephant seal. We talked about the elephant seal extensively in the last video. They may make other sorts of visual displays to fend off any other males. They may even have scent marking. We'll talk about another one in just a second. These sorts of delineations aren't just to keep away other males, but they may also be to attract the females. Hey look, I'm a strong guy. I've got this great territory. Come on over here. Importantly, these territories are not necessarily all equal. In fact, frequently they're not equal. They will vary quite a bit in quality. That you tend to see the strongest or largest males defending the optimal territories. Those territories may be optimal because they have a lot of food sources, or they may be optimal specifically because that's where the females are. Now a key part of this idea of delineating and defending a territory is intimidation. That you need to keep other males away from your territory. So, how do you do this? Well, animals have adopted a lot of different ways for doing this across species. So, in the case of the red wing blackbird that you see over here, he has these big red patches on his wings. And it's thought that those actually function for warding off potential threatening other males. The way that researches have studied this, or one of the ways they studied this, is to get stuffed red winged black birds, the ones that are dead, and set them up there. And what they found is if the stuffed one had its patch painted over, so that it was black, it was attacked very frequently by other males. In contrast, if there was this big red patch other males would basically leave it alone. So clearly, these red patches are enough to keep the other males away. Now, it's unclear why that's the case, and it could be this is some sort of indicator of ability, that maybe the males that have the big red patches tend to be healthier or something like that. We'll come back to that in the next video, but I just wanted to toss that out. But not all these signals are visual. If you look below here, we have the red eyed tree frog. Now what this does, is it will tend to vibrate branches. Again, it is thought that this sort of vibration is done to ward off other potential competing males who may want access to their territory. It's seeing or feeling these vibrations will ward off their rivals. But again, maybe this is an indication of ability as well. Everything we've talked about so far has been in the context of this sort of attack, dominance, warding off other things. Sometimes, animals like to be just a little bit sneaky instead. So here's the case we're looking at different strategies for getting access to males. And remember, not all males need to do exactly the same thing. It might not be your be your best at this and therefore that's the best thing to do, maybe there's different approaches. So here in this case, we're looking at a marine isopod. Now this particular isopod has three different forms. Now these are genetically determined. So there's what referred to as an alpha form, a beta form, and a gamma form. The alpha form is sort of the typical male that you would imagine. This is a big male, it fights off intruder males, it guards harems of females, things like that. The beta male's a little more sneaky. What it does is it actually mimics a female. It'll slip into the harem and sometimes have some activities there with the females in the harems, so that is one strategy. Another strategy is this gamma. These are very small ones, they tend to be very fast. Maybe they make juveniles, but they slip into these harems and slip out. Interestingly, all three of these forms persist in individual populations. Suggesting that on average relative to their frequency that they're equally fit. So this is a very interesting case where you have these three different approaches for getting access to the females. Now, everything we've talked about so far this video has been in terms of competition among males. So let's start looking at the other half. The females choose among these competing males, as well. It's not just that you beat out the other males, but the female has to actually want to mate with the male. So what it is that females are attracted to, because sometimes they will select amongst a pool of males that are all available. Well they may be attracted to various visible characters or displays. So maybe having this long tail isn't necessarily better for fighting off other males like in the tail here, but maybe instead it's actually attractive to the females. There's some evidence that's true. And maybe the females are attracted to good territories or nests. Therefore they can be provisioned better. Or it may be they're attracted to particular kinds of calls or smells, or something like that. Now, all these female preferences that exist will impose selection on the males, right? The males will respond to this. That those males that have the more attractive traits will get more matings, and therefore those traits will spread by natural selection. So let's look at one setting for where this female choice occurs. This is kind of an extreme one, but sometimes there are these competitive arenas for female choice where you'll have a set of males all displaying together, and almost in a competitive sort of way, and the females just wander through and choose a male. This is referred to as a lek. One of the most studied leks is that of the sage grouse, this particular bird down here. Again, all the males come together. They're doing their little dances and displays. The females wander through and they choose which ones they want. Some people have suggested that maybe bars are a human lek. I'm just kidding. I just wanted to toss that in there [LAUGH]. But anyway, you see the sort of competitive arena does exist. Now We've talked about competition as though it was something that was very obvious. But competition and choice may actually be less visible than is obvious. Now we've talked about competition and choice as being something that's very obvious. But in fact, both of those may be something that is not easily seen by an observer from the outside. So looking at this, this is a damselfly. And what the damselfly male does is, he has a very interestingly shaped male penis that goes into the female, and it actually will remove sperm from the previous mating before he puts his in or while he's putting his sperm in. So that's that kind of an unusual way of getting rid of the competition from a previous mating with the female. And lest you think this is exclusive to insects, in fact, this is actually thought to occur in primates. In fact, chimps have spines on their penises. This picture over here is from a marmoset, rather than a chimp, but see, these are the spikes on the marmoset's penis, on the side of his penis. And it's thought they use these penile spines to remove sperm from competitors when they mate with females. Fortunately, the human species has actually lost these spines, so we don't actually have those. Now we talk about this now in the context of getting rid of the sperm of another male, but sometimes you have direct competition of sperm happening inside the female as well. It doesn't have to be these dancing displays, but it could be something that's completely internal. Now there's a couple of different ways this could happen. One is that some animals will ejaculate non-fertilizing products first. And what they're trying to do is essentially wash out or somehow incapacitate the earlier sperm. So ejaculates have these offensive and defensive capabilities. Sometimes this offensive one is just wash out what's already in there. Other times you have this defensive capability where it will somehow try to keep the sperm that's in there. Along those lines, a lot of animals have what's referred to as a copulatory plug. This is true in rodents, and some insects, and reptiles as well. Where they'll put this gelatinous mass to try to basically plug in their sperm after they're mating. That's one strategy. Now, some animals will ejaculate very large quantities of sperm just to out-compete the possible past sperm. And some animals will, this is kind of a really sneaky one. They'll put anti-aphrodisiacs into the female. Well, what those anti-aphrodisiacs do is they make the female disinclined to mate again. It's basically manipulating the behavior of the female chemically by putting something in there. There's a lot of interesting strategies. Now you may be wondering about this little bird that's sitting here in the corner. That is the dunnock. It is a European bird. What happens in that case is the male, when he's about to mate with the female, he will peck at the cloaca of the female and basically try to force her to eject the sperm from previous matings. Not a very nice prospect. So, given what you've learned, in which of the following situations might you expect to see the most defenses to sperm competition evolve. Do you expect it in species in which females mate multiply? Do you expect it in species that have very little sexual dimorphism? And we talked about that in the previous video. Do you expect it in species that have males successfully defending harems for multiple seasons? Or do you expect it in species with effective seminal fluid anti-aphrodisiacs? Think about that for a second, and then try this question. Well, I hope that wasn't too difficult. The answer, as you saw, was indeed the first one. You have to multiple mating for sperm competition to be important. Why not these other ones? Well, if you have little sexual dimorphism, what does that mean? You may remember I said in the last video that sexual dimorphism correlates with the intensity of sexual selection, or basically the variance in mating success. So, if you have very little sexual dimorphism, it's basically almost like having monogamy, potentially. So as you approach monogamy, you're going to have less sexual dimorphism. So you really don't expect this to be the case where you're going to need sperm competition. You don't expect it where males are successfully defending harems for multiple seasons, because if they're defending the harems for multiple seasons then who are they competing with? Only themselves. And last, but not least, again if females have these anti-aphrodisiacs put into them, they're not going to mate again very readily and therefore you're not going to have the same sort of intensity of competition. So, let me close out this video by talking about why does female choice evolve. Well, there has to be some sort of benefit for this choice to evolve. Or, you think that would be true, at least. So, what are these benefits to the female for being choosy? Why don't they just mate with whatever male wins the fight? Why do they actually have to pick particular characters? And related to this is how do these preferences spread in natural populations? Well that will be the subject of our next video. Thank you and I hope you enjoyed this.

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